It is well known in the art of galvanic cell construction that certain cell systems, such as the lithium/oxyhalide cell system or the lithium/manganese dioxide cell system, and certain cell applications preferably require a hermetically sealed housing and, therefore, generally cannot utilize the resealable seals or vents known in the battery art.
Specifically, cell systems employing lithium, high vapor pressure organic solvents and inorganic liquid cathode/solvent materials are required to be hermetically sealed in order to prevent the exposure of the highly reactive and corrosive cell materials to the outside environment and to provide superior shelf life. The disadvantage or danger of utilizing a non-venting hermetic seal is that with the buildup of internal pressure within a cell, the force could reach a level that could cause the cell to disassemble. Although seals utilizing plugs that are ejected or melt after a predetermined pressure buildup or temperature level is reached have been employed with some success, they may not, in all cases, provide the quality of seal having the required hermeticity for the cell systems referred to above.
In consequence, hermetic seals or vents employing glass or ceramic materials have been employed in the past. For example, U.S. patent application No. 519,208 filed Aug. 1, 1983 discloses an hermetic seal for electrochemical cells comprising a flexible metallic cover sealed to the open end of a container housing the cell components. Such flexible cover has a radially inwardly disposed flange defining an opening through which a conductive terminal of the cell passes. The terminal is hermetically sealed to the cover by a layer of glass or ceramic material bonded to both the flange and the terminal. The flexible cover is formed such that a predetermined pressure buildup within the cell will cause the cover to flex sufficiently so as to fracture the glass or ceramic material, the bond between the cover and the glass or ceramic material, or the bond between the terminal and the glass or ceramic material, thereby forming a vent passage from the cell.
U.S. Pat. No. 4,233,372 discloses an hermetic glass-metal seal for electrochemical cells having a metallic central feedthrough which is sealed to a metal eyelet by a glass-to-metal seal. However, this seal is not intended to have a venting function.
U.S. Pat. No. 4,115,629 discloses a cell closure which includes a flexible metal member which has an aperture of predetermined area over which a non-polarized metallic disc of a size larger than the aperture is symmetrically positioned. A glass or ceramic layer hermetically bonds said disc over the aperture. The wall structure of this hermetic bond is formed so that a predetermined gas buildup within the cell will cause the wall to rupture thereby venting the gas from the cell.
U.S. Pat. No. 4,127,702 discloses a self-venting battery wherein an insulating material such as glass or ceramic is provided between and bonded to the terminals for electrically insulating them from one another and wherein the insulator is such that increasing pressure within the battery will cause the insulator to crack a sufficient amount to vent the pressure in the battery before causing the bond between the insulator and terminals to fail.
Hermetic seals which employ glass or ceramic materials and which are to serve a safety vent function desirably exhibit two characteristics, to wit: (1) good mechanical seal strenght; and (2) the ability to retain their hermeticity until a predetermined pressure has been reached and then to vent quickly upon encountering such pressure.
As is employed herein mechanical seal strength is defined as the outwardly directed force required to break the glass-to-metal or ceramic-to-metal bond. The desirability of good mechanical seal strength stems from the brittle nature of the glass or ceramic materials typically employed. Such brittleness may cause the seal to break during cell assembly as a result of the mechanical forces exerted on the seal during such assembly. Consequently, good mechanical seal strength is desirable in order to minimize the amount of scrap produced during cell manufacture.
Because a loss of the integrity of the hermetic seal may render a cell unmarketable or even damage the device in which the cell is inserted, it is desirable that an hermetic seal not vent at pressures well below that of said seal's predetermined venting pressure. Conversely, the structure of such seal should be such that the cell may vent quickly when the pressure inside the cell reaches such a predetermined limit in order to avoid disassembly of the cell. Thus it is important that a venting aperture be opened as quickly as possible once such critical pressure has been reached.
Accordingly, it is an object of this invention to provide a galvanic cell having a hermetic seal which seal possesses enchanced mechanical seal strength.
It is a further object of this invention to provide a galvanic cell having an hermetic seal which seal will permit excess pressure to be rapidly released from the cell's interior when pressure inside such cell exceeds a predetermined pressure.
The foregoing and additional objects will become more fully apparent from the description hereinafter and the accompanying drawings.